The invention relates to an apparatus on a carding machine for cotton, synthetic fibres and the like, in which there is at least one card flat bar having a card flat clothing.
It is known in a card flat bar for the card flat clothing, preferably wire hooks, to be arranged in a strip-like support layer, the clothing being attached to the card flat bar and lying opposite the clothing of a roller, for example the cylinder, and at least the regions of the card flat clothing that face the card flat bar comprising an iron material, especially of steel, with at least one magnetic means (element) being provided between the card flat bar and the regions of the card flat clothing that face the card flat bar.
The revolving card top of a carding machine is the crucial technological element for reducing the number of neps in the fibre material, for example, cotton, in its most highly opened state. In its interaction with the cylinder, the revolving card top loosens the fibre knots, it being necessary for the spacing to be as small as possible but for mutual contact between the clothings to be prevented. Contact results in unnecessary wear. Premature wear in turn results in a reduction in quality. The flexible revolving card top is also the only element which can be set to extremely narrow carding nips without significant adverse technological secondary effects.
In order reliably to manage extremely narrow carding nips, precision components are a prerequisite. The revolving card flats used simultaneously on a machine are referred to as a card flat set. The differences in dimensions from card flat to card flat in the card flat set should be as small as possible. Likewise, each individual card flat should have a high degree of evenness across the width of the machine. Because increased precision is always associated with increased cost, it is necessary to combine increased precision with optimum handling at an acceptable cost. In practice, the clothings are clipped onto the card flats using enormous forces. The clipping-on operation, which has to be made reversible for re-clothing, has an adverse effect on precision and is not possible without destruction of the clothing.
In a known apparatus (DE 10 2006 005 605 A), the card flat clothing is adhesively bonded, in a tolerance compensating manner, to a metal backing sheet and is held in the revolving card top by a planar magnetic strip. The magnetic strip itself is in turn adhesively bonded, in a tolerance compensating manner, to the card flat bar. The magnetic force absorbs the process forces during the carding process with a high degree of reliability. As a result, many of the disadvantages of the old clip-on card flat system have been eliminated. The card flat sets have a high degree of precision even without an additional grinding process. Handling during re-clothing is optimum, because the clothing can be demounted, without being destroyed, using a single movement. The new clothing can be inserted again just as quickly.
The magnetic connection is a force-based connection. If a threshold force opposite to the attractive force of the magnet is applied, the clothing strip becomes detached from the card flat bar. The threshold force is such that the normal process forces can be transmitted with a high degree of reliability. This has been demonstrated by a large number of practical tests and experiments. The “old” mounting technique using clips was an interlocking connection. That connection could be broken only by overcoming the rigidity of the component. The forces necessary for that purpose are in turn a multiple greater than the threshold force of the “new” magnetic connection.
If operating conditions that can be considered abnormal then arise in a carding machine, forces can develop which exceed the threshold force of the magnet but still lie significantly below the connection strength of the clip-on technique. Abnormal operating conditions arise when the nips used are too narrow; when the fibre/clothing combination has been incorrectly selected and therefore cylinders become clogged; when, as a result of fibres that are difficult to process, temperatures suddenly rise very rapidly and there is substantial contact between clothings; when operators do not recognise the abnormal operating conditions in good time and allow the machines to continue running, and so on. It can also happen that an unusually large or solid disruptive element, for example a trash particle, fibre knot or the like, projects at least partly beyond the circle of tips of the cylinder and thus exerts undesirable pressure on the forwardly arranged regions (front regions) of the clothing of at least one card flat bar. In summary, there are situations which occur extremely rarely (exceptional cases) but give rise to enormous adverse forces.
In normal operation, the magnet absorbs all the operating forces and provides for precision support. In an abnormal operating state, the interlocking connection safeguards against contact with the cylinder clothing.